Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 meters or more) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 meters)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. As a result the need for high levels of performance from the layers of the flexible pipe body is increased.
Flexible pipe may also be used for shallow water applications (for example less than around 500 meters depth) or even for shore (overland) applications.
In flexible pipes there are often used polymer layers, such as PVDF (polyvinylidene fluoride), that may be formed by extrusion. Most polymers will have a certain maximum allowable strain above which the risk of damage to the material is much greater. In flexible pipes where a polymer layer lies adjacent an armour layer (such as a polymer barrier layer adjacent a metallic pressure armour layer), the polymer layer may be subjected to quite severe non-uniform, highly localised strain. This is because the armour layer is usually formed from interlocking wires of certain cross section, and there are certain gaps between adjacent windings. The polymer layer tends to deform and creep into the gaps when under pressure.
For example, it will be understood that a flexible pipe is an assembly of a portion of a pipe body and one or more end fittings in each of which a respective end of the pipe body is terminated. FIG. 1 illustrates how pipe body 100 may be formed from a combination of layered materials that form a pressure-containing conduit. It is to be noted that the layer thicknesses are shown for illustrative purposes only.
As illustrated in FIG. 1, a pipe body may include an optional innermost carcass layer 101. The carcass provides an interlocked construction that can be used as the innermost layer to prevent, totally or partially, collapse of an internal pressure sheath 102 due to pipe decompression, external pressure, and tensile armour pressure and mechanical crushing loads. As is known in the technical field, there are ‘smooth bore’ operations (i.e. without a carcass) as well as ‘rough bore’ applications (with a carcass). The carcass layer may be formed from helically wrapped metallic tape having a shaped cross section to allow interlocking of adjacent wrapped tape portions.
The internal pressure sheath 102 acts as a fluid retaining layer and comprises a polymer layer that ensures internal fluid integrity. It is to be understood that this layer may itself comprise a number of sub-layers. It will be appreciated that when the optional carcass layer is utilised the internal pressure sheath is often referred to by those skilled in the art as a barrier layer. In operation without such a carcass (so-called smooth bore operation) the internal pressure sheath may be referred to as a liner.
In addition, and not shown in FIG. 1, there may also be included a wear layer between the carcass layer and internal pressure sheath. The wear (or sacrificial) layer may be a polymer layer (often extruded but sometimes in tape form) intended to provide a smoother surface or bed for the barrier layer to be extruded onto than would be the case over the carcass layer, which may have undulations and gaps between wraps; this smoother wear layer surface may allow the barrier layer to experience higher levels of general strain (extension) as a result of bending and pressure because what local stress concentrations remain are relatively small and insignificant. Without such a wear layer the extruded polymer barrier may exhibit an undulating inner surface with protruding cusps of material that have naturally flowed into gaps in the carcass layer during the extrusion process; these cusps act as stress concentrators when the polymer is strained.
An optional pressure armour layer 103 is a structural layer that increases the resistance of the flexible pipe to internal and external pressure and mechanical crushing loads. The layer also structurally supports the internal pressure sheath, and typically consists of an interlocked construction of wires with a lay angle close to 90°.
The flexible pipe body also includes an optional first tensile armour layer 105 and optional second tensile armour layer 106. Each tensile armour layer is used to sustain tensile loads and internal pressure. The tensile armour layer is often formed from a plurality of metallic wires (to impart strength to the layer) that are located over an inner layer and are helically wound along the length of the pipe at a lay angle typically between about 10° to 55°. The tensile armour layers are often counter-wound in pairs.
The flexible pipe body shown also includes optional layers of tape 104 which help contain underlying layers and to some extent prevent abrasion between adjacent layers.
The flexible pipe body also typically includes optional layers of insulation 107 and an outer sheath 108, which comprises a polymer layer used to protect the pipe against penetration of seawater and other external environments, corrosion, abrasion and mechanical damage.
Each flexible pipe comprises at least one portion, sometimes referred to as a segment or section of pipe body 100 together with an end fitting located at at least one end of the flexible pipe. An end fitting provides a mechanical device which forms the transition between the flexible pipe body and a connector. The different pipe layers as shown, for example, in FIG. 1 are terminated in the end fitting in such a way as to transfer the load between the flexible pipe and the connector.
FIG. 2 illustrates a riser assembly 200 suitable for transporting production fluid such as oil and/or gas and/or water from a sub-sea location 201 to a floating facility 202. For example, in FIG. 2 the sub-sea location 201 includes a sub-sea flow line. The flexible flow line 205 comprises a flexible pipe, wholly or in part, resting on the sea floor 204 or buried below the sea floor and used in a static application. The floating facility may be provided by a platform and/or buoy or, as illustrated in FIG. 2, a ship. The riser assembly 200 is provided as a flexible riser, that is to say a flexible pipe 203 connecting the ship to the sea floor installation. The flexible pipe may be in segments of flexible pipe body with connecting end fittings.
It will be appreciated that there are different types of riser, as is well-known by those skilled in the art. Embodiments of the present invention may be used with any type of riser, such as a freely suspended (free, catenary riser), a riser restrained to some extent (buoys, chains), totally restrained riser or enclosed in a tube (I or J tubes).
FIG. 2 also illustrates how portions of flexible pipe can be utilised as a flow line 205 or jumper 206.
In accordance with industry regulations, all flexible pipe structures must undergo a factory acceptance test (FAT) prior to sale. This involves pressurising a pipe bore with a fluid such as water at 1.5 times the usual pressure of use. The water is thus a pressurising medium.
The application of internal pressure (i.e. pressure from within the bore) to the pipe produces radial expansion in all layers and this is when a polymer layer undergoes deformation and tends to creep into the gaps of an overlying armour layer. At high pressures (about 8000 psi/55 MPa or more), the resultant strain distribution within the polymer layer can be highly localised at the areas around the gaps, and the polymer material may deform by cavitation rather than plastic flow. This can in turn result in the formation of microcrazing or microcracking on the radially inner surface of the polymer layer. During any subsequent loading (such as the loading experienced during normal use in transporting production fluids) this microcrazing may then extend to form longer/deeper cracks throughout the polymer layer. This increases the risk of failure of the polymer layer and may ultimately lead to loss of pressure containment, having an adverse effect on the lifetime of a flexible pipe.
According to a first aspect of the present invention there is provided a method of producing a flexible pipe body, comprising: providing a tubular layer; and directing a chemical reagent towards a surface portion of the tubular layer, wherein the tubular layer comprises an extruded polymer, and wherein the chemical reagent is suitable for changing one or more physical or mechanical property of a proportion of the extruded tubular layer thickness.
According to a second aspect of the present invention there is provided a flexible pipe body formed by a process comprising: providing a tubular layer; and directing a chemical reagent towards a surface portion of the tubular layer, wherein the tubular layer comprises an extruded polymer, and wherein the chemical reagent is suitable for changing one or more physical or mechanical property of a proportion of the extruded tubular layer thickness.
According to a third aspect of the present invention there is provided a flexible pipe body for transporting oil or gas or other such fluid from a sub-sea location, comprising: a tubular layer comprising a polymer, wherein the tubular layer has a radially inner edge portion or a radially outer edge portion that has an elastic modulus that is lower than the remainder of the tubular layer.
According to a fourth aspect of the present invention there is provided a method substantially as herein described with reference to the drawings.
According to a fifth aspect of the present invention there is provided a flexible pipe body substantially as herein described with reference to the drawings.
Certain embodiments of the invention provide the advantage that a flexible pipe body is provided that has been treated to reduce, inhibit or prevent microcrazing.
Certain embodiments of the invention provide the advantage that a method of treating a flexible pipe body is provided to reduce, inhibit or prevent microcrazing that can be conveniently incorporated into current pipe manufacturing processes.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
In the drawings like reference numerals refer to like parts.